Such apparatus may be used in electric meters for the conversion of a signal, which is proportional to an electric power or load, (i.e. a current/voltage product) into a proportional pulse frequency. Illustratively, the frequency refers to the pulse frequency of a signal comprising a train rectangular pulses.
An apparatus of such kind is known from U.S. Pat. No. 4,124,821. There the principle of periodic polarity conversion is used to eliminate an offset-voltage. Normally this offset-voltage, being part of an input signal, cannot be eliminated and depends on time and temperature and thus directly effects the exactness of the signal to frequency conversion as an error in linearity. In the periodic polarity conversion method, the signal that is to be converted to a proportional frequency and the transmission direction of the apparatus are periodically and simultaneously reversed, so that both sign changes mutually cancel each other. However the offset voltage, which only changes sign with the transmission direction, is alternately added to and subtracted from the signal to be converted during successive half period of a switch signal. Thus the integration of the offset-voltage over an integral number of periods of the switch signal results in zero if both half-periods of each switch cycle last equally long.
Since normally the integration capacitor has a non-zero voltage at the time of polarity reversal, errors due to this non-zero votlage are introduced into the conversion result. When the time of measurement is long, these errors do not necessarily have a negative influence on the average of the output pulse frequency. But in any case these errors lead to momentary fluctuations, i.e. to a modulation of the pulse frequency. In the prior art in order to avoid this, the time of polarity reversal is synchronized with the output signal of a comparator, which senses the capacitor-voltage. Thus the polarity reversal always happens at the time of zero crossing of the capacitor voltage. However this solution results in the two half periods not always being the same. Instead they only last an equally long time in a statistic average. Again this leads to an irregular modulation of the output frequency of the apparatus and makes its calibration difficult. Calibration can only be accomplished after an unsatisfactory long time period.
It is the object of this invention to provide an apparatus and method for converting an electrical signal into a proportional pulse frequency in which there is avoided modulation of the output pulse frequency resulting from polarity switching cycles having half periods of unequal duration and capacitor voltages which might be non-zero at the time of polarity reversal so that a faster and more precise calibration of the apparatus is possible.